Electrostatic recording medium

ABSTRACT

There is provided an improved electrostatographic product having an electroconductive supporting sheet with a recording layer formed on the supporting sheet, said recording layer comprising a mixture of an insulating polymer and a ferroelectric powder having a Curie temperature above room temperature. The product will accept a persistent electrostatic charge pattern by the application of a low voltage to the recording layer.

United States Patent 1191 I Obu et al. Feb. 19, 1974 [54] ELECTROSTATIC RECORDING MEDIUM 3,442,699 5/1969 Dalton 117/201 [75] Inventors: Makoto Obu; Tadamitsu Uchiyama,

both of Tokyo, Japan Primary Exammer-Joseph W. Hartary Assigneel Kobllshiki Kaisha Ricoh, y Attorney, Agent, or FirmCooper, Dunham, Clark, Japan Griffin 8L Moran [22] Filed: July 28, 1972 [21] Appl. No.: 276,137

' I [57] ABSTRACT [30] Foreign Application Priority Data Aug. 25, 1971 Japan 46-64889 Th r i provided n improved electrostatographic product having an electroconductive supporting sheet [52] US. Cl 346/135, 117/201, 252/62.63, with a recording layer formed on the supporting sheet,

346/74 ES said recording layer comprising a mixture of an insu- [51] Int. Cl. G0ld 15/34 lating polymer and a ferroelectric powder having a [58] Field of Search. 346/135, 74 ES; 117/201, 221; Curie temperature above room temperature. The 252/520, 521, 62.63, 62.53, 62,54 product will accept a persistent electrostatic charge 1 pattern by the application of a low voltage to the re- [56] References Cited cording layer.

UNITED STATES PATENTS 2,664,043 12/1953 Dalton 346/135 X 4 Claims, N0 Drawings ELECTROSTATIC RECORDING MEDIUM BACKGROUND OF THE INVENTION Electrostatographic products for use in facsimile recording comprise an electroconductive supporting sheet and a recording layer formed on the supporting sheet. An electrostatic charge pattern corresponding to an image to be produced is formed on the recording layer by discharge from conducting surfaces (electrodes). The electrostatic charge pattern is converted into a visible image by the deposition of electrically attractable particles. More specifically, the electrostatic charge pattern is produced on the recording layer by applying a direct current voltage between two electrodes separated by the recording layer. The electroconductive supporting sheet is utilized as one electrode. The other electrode is a metallic needle-like electrode or a metallic convex electrode. The electrostatic charge pattern is produced on the recording layer by controlled tracking of the needle-like electrode or from a corresponding figure on the metallic convex electrode.

The electroconductive supporting sheet of the electrostatographic product may be prepared, for example,

by applying an electroconductive substance such as carbon powder or metal powder, or a hygroscopic substance such as water glass, magnesium chloride or glycerin to the surface of a normally flexible sheet, suitably paper or a polymer film such as a polyester film. Alternatively, a metal coat such as aluminum or other conductive metal may be deposited on the sheet by vacuum evaporation.

The recording layer normally comprises a polymeric,

dielectric material having high electric resistance of more than ohm-cm. Typically, acrylic and methacrylic polymers, or copolymers of vinyl chloride and vinyl acetate or methyl styrene and styrene are employed. The recording layer may be formed by applying a solution of dielectrics in an organic solvent to a surface of the electroconductive supporting sheet and drying it to evaporate the solvent. The thickness of the recording layer is usually about 10 microns.

Conventional electrostatographic products thus prepared are not completely satisfactory since a high voltage, which may be as high as 300 V to 700 V, is required to produce the electrostatic charge pattern. Moreover, the 1 charge pattern thus produced has low persistence.

It is an object of the present invention to provide an improved electrostatographic product in which an electrostatic charge pattern having prolonged persistence can be produced on a recording layer by applying a low voltage.

SUMMARY OF THE INVENTION The present invention relates to an improved electrostatographic product on which I an electrostatic charge pattern of long duration can be produced on the recording layer by applying a voltage as low as 50 V to 300 V. 1

The electrostatographic product of this invention is an electroconductive supporting sheet having a recording layer which comprises a mixture of an insulating polymer and' powder of a ferroelectric substance.

The electroconductive supporting sheet is prepared in the usual manner as described above' and has an electric resistance of less than 10 ohm-cm.

The insulating polymer may be any of the natural or synthetic polymers or resins which have electric resistance of more than 10" ohm-cm. of the type normally utilized in the preparation of conventional products which are stable, that is, they do not decompose or melt under the applied voltage. These may include, for example:

styrenated alkyd resins, phenolic resins, polyure thane resins, silicone resins, acrylic resins, urea resins, vinyl chloride-vinyl acetate copolymers, styrenebutadiene copolymers, methylstyrene-styrene copolymer, butadiene-methyl-methacrylate copolymers, butadiene acrylonitrile copolymers, ethyl cellulose, cellulose acetate, polyvinyl chloride and polyvinyl acetate. The ferroelectric substances used in this invention are characterized as having a Curie temperature above room temperature. Typically, it is at least about 40C.

It has been found that with ferroelectric substances having a Curie temperature at room temperature that is about 20C., transition of spontaneous polarization occurs at room temperature and with the result that a stable electrostatic charge. pattern cannot be obtained at room temperature.

While a number of ferroelectric products can be used in this invention, the preferred products from considerations of economy, availability and results are:

Ferroelectric Substances Curie Temperatures (C.)

Potassium Niobate KNb0 435 Barium titanate BaTi0 l20 Lead titanate PbTiO 490 Lithium niobate LiNbO; 1210 Lithium tantalate LiTaO: 665 Lead metaniobate Pb(Nb0;) 560 Lead metatantalate Pb('l'a0;,) 265 Sodium nitrite NaNO, I63 Potassium nitrite KNO, 47 Rubidium tantalate RuTaO, 247

The ferroelectric substances as used are normally in the form of powders or particles with a size of from about 0.1 to 30 microns, although some variation from this range can be tolerated. When the powder having very small particle size appreciably less than 0.1 microns is used, the electrostatic charge pattern may vary somewhat from optimum. On the other hand, with a particle size of more than 30 microns, the recording layer is unnecessarily thick.

The amount of ferroelectric substance in the recording layer is usually from about 5 to 50 parts by weight of the powder to 1 part by weight terial.

An electrostatographic product of the present invention may be prepared as follows:

Particles of the selected ferroelectric substance are dispersed in a solution of the dielectric polymer in an organic solvent such as toluene by means of a homomixer. The dispersion thus obtained is applied to the surface of an electroconductive supporting sheet by any conventional means. The use of a wire-bar is especially preferred. The solvent is evaporated to fonn a recording layer having thickness of 5 microns to 50 microns on the supporting sheet.

The following non-limiting examples are given by way of illustration only:

EXAMPLE 1 v A dispersion of powder of barium titanate was pre of the insulating ma- Barium titanatc (powder having particle size of 0.5 to 20 microns) Acrylic resin (sold by Japan Reichhold Chemicals lncv under the mark of A-40l) Toluene The dispersion thus prepared was coated on a surface of aluminum-coated polyethylene terephthalate film by a wire-bar so as to obtain a recording layer having thickness of 20 microns (after drying).

The dielectric constant of the recording layer was about 30. An electrostatic charge pattern was produced on the recording layer by using the aluminum coating as a positive electrode and a needle-like electrode as a negative electrode and applying a D.C. voltage of about 70 V across the recording layer.

The electrostatic charge pattern thus produced was developed into a visible image by the application of a developer which has been prepared by dispersing a mixture of l g of carbon black and 20 g of styrenated alkyd resin in 1 litre of a paraffinic hydrocarbon (sold by Esso-Standard Oil Co. under the trademark of lsopar H). r

The charge pattern persisted for about 10 minutes, appreciably longer than with conventional products. This persistence of the charge pattern can be detected by the formation of a visible image with a developer.

EXAMPLE 2 Sodium nitrite (powder having particle size of 0.5 to 10 microns) Vinyl chloride (8 mols)- vinyl acetate (2 mols) copolymer (having molecular weight of about 20000) Toluene lOg l00 ml The electrostatographic product thus prepared gave the same results as that of-Example 1 upon application of D.C. voltage of to the recording layer.

EXAMPLE 3 An electrostatographic product was prepared by repeating the same procedure as-that of Example 1 except that a recording layer having thickness of 20 microns was formed on the electroconductive supporting.

sheet by using the following ingredients in place of the ingredients of Example 1:

lOOg

I00 ml The'electrostatographic product thus prepared gave the same results as that of Example 1 upon application of DC voltage of 60 V to the recording layer.

1. An electrostatographic product of the type in which an electrostatic charge pattern corresponding toan electrostatic image to be reproduced is formed on a recording layer by discharge from electrodes'and thereafter said image is converted to a visible image by deposition of electrically attractable particles, said product comprising an electroconductive supporting sheet and a recording layer, said recording layer containing from about 5 to 50 parts by weight of a ferroelectric powder having a Curie temperature-of at least about 40C. dispersed in one part by weight of a stable insulating polymer, said electrostatographic product being characterized bythe ability to accept an electrostatic charge pattern upon application of a charge of from 50 to 300 volts. I

2. An electrostatographic product according to claim 1 wherein the ferroelectric powder is one selected from the group consisting of potassium niobate, barium titanate, lead titanate, lithium niobate, lithium tantalate, lead metaniobate, lead metatantalate, sodium nitrite, potassium nitrite and rubidium tantalate.

3; An electrostatographic product according to claim 1 wherein particle size of the ferroelectric powder is from about 0.1 microns to 30 microns.

4. An electrostatographic product according to claim 1 wherein the insulating. polymer is one selected from the group consisting of acrylic polymers, vinyl chlo:

ride-vinyl acetate copolymers and methylstyrenestyrene copolymers. g 

2. An electrostatographic product according to claim 1 wherein the ferroelectric powder is one selected from the group consisting of potassium niobate, barium titanate, lead titanate, lithium niobate, lithium tantalate, lead metaniobate, lead metatantalate, sodium nitrite, potassium nitrite and rubidium tantalate.
 3. An electrostatographic product according to claim 1 wherein particle size of the ferroelectric powder is from about 0.1 microns to 30 microns.
 4. An electrostatogrAphic product according to claim 1 wherein the insulating polymer is one selected from the group consisting of acrylic polymers, vinyl chloride-vinyl acetate copolymers and methylstyrene-styrene copolymers. 